Polysaccharide Storage Myopathy (PSSM) is a debilitating muscle disease in many and diverse breeds of horses. Previous data indicates that approximately 10% of Quarter Horses and 36% of Belgian draft horses are affected. Clinical signs vary, but can range from muscle atrophy and progressive weakness in Draft horse breeds, to acute post-exercise muscle cramping and cell damage in Quarter Horses and other breeds. All forms of PSSM in horses are highly associated with deposits of an abnormal polysaccharide in skeletal muscle fibers that are demonstrated by histochemical staining of muscle biopsies. PSSM is also characterized by as much as four times the normal level of glycogen in skeletal muscle. Mutations in genes of glucose and glycogen metabolism are known to cause various types of glycogen storage diseases (glycogenoses) in humans and animal species, of which several histologically resemble PSSM. However, none of these genes appear to be responsible for equine PSSM.
The current diagnosis of PSSM in horses is based on clinical signs of muscle cramping or progressive atrophy (depending on the breed), often with elevated serum levels of muscle enzymes, combined with the histopathology finding of abnormal polysaccharide in thin sections cut from skeletal muscle biopsies.
Muscle biopsies are invasive, require skilled veterinary personnel to collect, are relatively expensive for the owner, and take a skilled muscle histopathologist to interpret. Further, although the muscle biopsy analysis has been a highly reliable diagnostic tool, it is not now 100% specific or sensitive, and can never hope to be.
Therefore, despite the foregoing, there is a need in the art for additional diagnostic tests for diagnosing PSSM in horses.